Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate, and a latent electrostatic image is created by projecting the penetrating radiation such as X-rays or gamma rays through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latent electrostatic image on the plate surface with fine powdered particles electrically charged opposite to the latent electrostatic pattern on the plate. The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry, and no dark room is necessary.
Xeroradiography in recent years has been utilized to detect breast cancer in women. In the examination of breasts wherein soft tissue comprises most of the breast area, xeroradiography or xeromammography, as it is generally called, provides greater resolving power than the conventional roentgenographic film, and greater image detail is achieved. A wide range of contrast is seen on the xerographic plate as compared to the conventional roentgenographic films so that all structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumor calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than prior non-photoconductive X-ray techniques.
A factor which has influenced some radiologists to limit xeroradiography applications to the examination of breasts and the extremities (i.e., hands and feet) is the radiation dosage required in examining chests, skulls, hips, etc. as compared to conventional screened X-ray film.
Since a substantial portion of the generated X-ray radiation will remain in the body tissues of the target area, radiologists have been reluctant to use X-ray systems requiring radiation dosage levels which, although below the recognized minimum safety level, is still higher than that utilized in conventional screened X-ray film.
It would therefore be desirable to provide a technique compatible, for example, with the automated xeroradiographic system described in U.S. Pat. No. 3,650,620, which will reduce the X-ray dosage required to examine particular areas of the human anatomy, such as the chest, while at the same time not reducing the relative information capacity of the developed images.
A system which provides a technique compatible with an automated xerographic system and which requires a reduced X-ray dosage to examine particular areas of the human anatomy is described in copending application Ser. No. 448,128, filed Mar. 4, 1974, now U.S. Pat. No. 3,914,609, issued Oct. 21, 1975 and assigned to the assignee of this application. The technique described therein is operative in either a positive or negative development mode. Although the negative development mode provides satisfactory images, an improved technique for providing higher quality images in the negative mode would be desirable.